Tuesday, February 14, 2012

The song of Archobollus musicus

ResearchBlogging.org Within a sparsely populated coniferous forest, night is quickly approaching.  As twilight pushes on and the sun slowly disappears from the horizon, the nocturnal singers of the forest begin to awaken.  Joining the choir of amphibians and other mistrals of the night a lone katydid begins to play its chirping melody.

Staying low to the ground, the katydid hides amongst the foliage of the giant fern Caniopteris.  It sings its song as it scrapes its wings together, passing one along the rides of the other.  Quieting down only when a small mammal or dinosaur becomes too curious about the source of the ethereal sound.  A scene that would play out nightly for millions of years.

For, like the katydid, the forest died long ago.  But in the mid-Jurassic, both thrived in what is now northwest China.  This particular katydid, known as Archabollus musicus belonged to a family known as Haglidae, a group of Orthopterans that existed from the early Triassic until ultimately becoming extinct in the late Cretaceous.

Orthoptera is the order of insects that includes all crickets, grasshoppers, locusts and katydids.  Like so many of its now living relatives Archabollus musicus produced sound through the process of stridulation (the action of creating sound by the rubbing together of the wings or the legs).  This process can produce one of two types of sound, resonant or non-resonant.  The non-resonant producing insects create a wide variety of tones where as the resonant, or musical insects, produce pure-tones.  A. musicus was a member of the later and more ancestral group, producing a resonant sound at 6.4 khz, well within the threshold for human hearing.

One might wonder how we can know the specific frequency of sound produced by an insect that lived 165 million years ago.  The answer comes from a particularly well preserved fossil recently unearthed.  In it, the wing is so well preserved that the stidulatory file, the series of ridges along one of the wings that the other wing scrapes along to produce sound, is almost perfectly preserved.

As the plectrum, the appendage that scrapes along the file, passes over each tooth, the resulting vibrations produce sound.  The shape and spacing of the teeth dictate the kind of sound that will be produced, just like how the sound of running your thumb over different combs produce different sounds.  And just like with combs, how fast the plectrum passes over the stidulatory file affects what kind of sound will be produced.  Based on the shape and size of the teeth, there is a specific speed that produces the optimum tone.

The fossil wing of Archabollus musicus next to a representation of the stridulatory file.
Using this knowledge, Fernando Montealegre-Zapata of the University of Bristol in the UK and his colleges Jun-Jie Gu, Daniel Roberts, Michael S. Engel, Ge-Xia Qiao, and Dong Ren were able to begin work on reproducing the sound of this long extinct katydid.

From the cavernous expanse of deep time, a sound that has not been heard upon this Earth for 165 millions years begins to resonate.  It is a sound both familiar and haunting.  While it is a simple sound, it is one that deserves respect and elicits awe.  It is the sound of our own curiosity and ingenuity.  It is the sound of beauty.

Gu, J., Montealegre-Z, F., Robert, D., Engel, M., Qiao, G., & Ren, D. (2012). Wing stridulation in a Jurassic katydid (Insecta, Orthoptera) produced low-pitched musical calls to attract females Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1118372109
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